Electric Sail Concept Moves Forward

by Paul Gilster on August 11, 2014

Just how we follow up on the investigations of New Horizons remains an open question. But we need to be thinking about how we can push past the outer planets to continue our study of the heliopause and the larger interstellar environment in which the Sun moves. I notice that Bruce Wiegmann, writing a precis of a mission concept called the Heliopause Electrostatic Rapid Transit System (HERTS) has drawn inspiration from the Heliophysics Decadal Survey, which cites the need for in situ measurements of the outer heliosphere and beyond.

It’s good to see a bit more momentum building for continuing the grand voyages of exploration exemplified by the Pioneers, the Voyagers and New Horizons. I often cite the Innovative Interstellar Explorer concept developed at Johns Hopkins (APL), which targets nearby interstellar space at a distance of over 200 AU, but whether we’re talking about IIE or Claudio Maccone’s FOCAL mission or any other design aimed at exiting the Solar System, the key problem is propulsion. Weigmann’s team at Marshall Space Flight Center has been awarded a Phase I grant from NASA’s Innovative Advanced Concepts office to work on a dramatic solution.

The Heliopause Electrostatic Rapid Transit System involves a sail and thus propellant-less propulsion, but it’s not the conventional solar sail that uses the momentum provided by solar photons. The nomenclature is confusing, because the electric sail HERTS is designed around would interact with the solar ‘wind,’ which is not made up of photons at all but a stream of charged particles that flows constantly though erratically from the Sun at high velocity. A spacecraft riding the solar wind could, by some calculations, move between five and ten times faster than our best outer-system result so far, the 17.1 km/sec Voyager 1.

Wiegmann explains the principle at play in the precis:

The basic principle on which the HERTS operates is the exchange of momentum between an array of long electrically biased wires and the solar wind protons, which flow radially away from the sun at speeds ranging from 300 to 700 km/s. A high-voltage, positive bias on the wires, which are oriented normal to the solar wind flow, deflects the streaming protons, resulting in a reaction force on the wires—also directed radially away from the sun. Over periods of months, this small force can accelerate the spacecraft to enormous speeds—on the order of 100-150 km/s (~ 20 to 30 AU/year). The proposed HERTS can provide the unique ability to explore the Heliopause and the extreme outer solar system on timescales of less than a decade.

If you’re an old Centauri Dreams hand, you’ll recognize the HERTS sail as the offspring of Pekka Janhunen (Finnish Meteorological Institute), whose concept involves long tethers (perhaps reaching 20 kilometers in length) extended from the spacecraft, each maintaining a steady electric potential with the help of a solar-powered electron gun aboard the vehicle. As many as a hundred tethers — these are thinner than a human hair — could be deployed to achieve maximum effect. While the solar wind is far weaker than solar photon pressure, an electric sail with tethers in place is still efficient, according to Janhunen’s calculations, and can create an effective solar wind sail area of several square kilometers.

Image: A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions “see” the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or “flying” the resulting spacecraft electrically. Credit: Artwork by Alexandre Szames. Caption via Pekka Janhunen/Kumpula Space Centre.

MSFC’s Advanced Concepts Office has been studying the feasibility of the Janhunen sail during the past year, finding that the electric sail is able to reach velocities three to four times greater than any realistic current technology including solar (photon) sails and solar electric propulsion systems. Because we are dealing with a stream of particles flowing outward from the Sun (and because the electric sail can, like a solar sail, be ‘tacked’ for maneuvering), we are looking at a fast interplanetary propulsion system that avoids the deployment issues faced by large solar sails using photon momentum for their push. Deploying reels of tethers is, by comparison, straightforward.

Both photon-pushed sails and those riding the solar wind are limited by distance from the Sun, but the electric sail may have applications in future interstellar missions nonetheless. If we accelerate a (non-electric) sail by the use of a laser or microwave beam up to a small percentage of the speed of light, we could slow it down upon arrival by using the solar wind from the destination star, interacting with a tether system deployed as the spacecraft enters the new system. Having decelerated, the spacecraft could then use electric sail technology for exploration. Janhunen has explored the concept for electric sails (though not yet in detail), but an idea like this was also broached by Robert Zubrin and Dana Andrews for magnetic sail deceleration in 1990.

Does anyone have any links to the status and any results of the testing of the concept with EST-1 and Aalto-1 nanosatellites?

We’ve talked about using beams to accelerate solar sails. Just how feasible would it be to accelerate electric sails with charged particle beams in a similar manner, or is this unfeasible by the very nature of the beams? (I assume a plasma of protons and electrons to create an overall neutral beam would be required).

In a similar vein, is there any advantage in trying to capture “gusts” from CMEs (natural or even deflected) or are they too transient to make any difference?

The proposed Innovative Interstellar Explorer (IIE) uses currently available propulsion methods, but has narrow launch windows that are 12 years apart (the next is 2026) and a 30-year flight time to 200 a.u. I (and I suspect many here) will be 100+ years old by the time the next possible IIE mission would be completed. Even if it takes 20 years to develop an electric sail the shorter flight time will more than make up for it, and give some of us a chance of seeing it through. Go for it!

Re: Over periods of months, this small force can accelerate the spacecraft to enormous speeds—on the order of 100-150 km/s (~ 20 to 30 AU/year).

Now that is what I call living off the land. I’d imagine that due to public and federal sentiment against Project Orion style nuclear bomb driven craft and public aversion to launching nuclear fission fuel in space. we are going to need a lot more new and innovative concepts like the electric sail.

Besides, the illustration looks pretty. I hope they bring this technology to practice.

‘We’ve talked about using beams to accelerate solar sails. Just how feasible would it be to accelerate electric sails with charged particle beams in a similar manner’

There is a limit to how charged a material can be to reflect the charged particles effectively, if the particles are going to fast they will not have enough time to dump momentum into a large enough amount of material and will punch right through.

‘In a similar vein, is there any advantage in trying to capture “gusts” from CMEs (natural or even deflected) or are they too transient to make any difference?’

If the deployment of the sail is quick there is no reason why the craft can’t wait until the right CME comes along, electric sails could potentially open much quicker than a solar sail.

-Bursting out of the Sun at a blazing velocity of 4.4 million mph (nearly 2,000 kilometres per second)-

Even the LHC, the most powerful generator of proton beams extant, would not provide much momentum for the HERTS scheme. And it’s not even space-based. Perhaps we should work with the sun rather than attempt to recreate it; and by that I have in mind a focusing scheme to concentrate a part of the solar wind into the semblance of a beam. Magnets in space! At least cryo-cooling will be minimal for the superconductors.

The fundamental problem of the E-sail as with the “conventional” solar sail is what mechanism can deploy the wires or sail mesh in a multi-kilometer deployment that is a consist, symmetric and durable?

This is probably a great way of launching small probes on quick trajectories to the outer solar system, but to be clear (if I’m understanding correctly), it’s low thrust, even by comparison to a solar-photon sail. Thus it wouldn’t be intended for use with heavier manned vehicles. Also, it doesn’t seem to have a means of decelerating, so it will be doing very high-speed flybys of whatever objects it targets. Still, I’ll bet it could collect a lot of interesting data as it passes through the outer Solar System.

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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